Production of cheese today is a big business involving the processing of large quantities of milk every day. In order to attain the desired production level it is necessary that the milk clot in a specific period of time. Before clotting can occur there must be lactic acid production. Lactic acid is produced by the action of starter cultures which are lactic acid bacteria which both produce the lactic acid and provide enzyme systems for flavor development during curing. Failure to produce enough lactic acid within the time schedule results in economic loss either through dumping the vat full of milk or by degradation of the cheese quality.
Starter cultures are blends of genetically distinct strains or subclasses or organisms primarily within two classes of lactic acid producing bacteria, including Streptococcus cremoris and Streptococcus lactis. All culture strains are subject to attack by bacteriophages which are bacterial viruses which can attack bacterial cells, multiply within the bacterial cell and ultimately destroy it. These bacterial viruses are also genetically distinct and this enables the viruses to differentiate between the various strains of starter organisms . . . thus certain viruses will attack only particular strains of S. cremoris and S. lactis.
Phage is always present. Obviously a starter culture must be resistant to the phage in order to function properly. A given blend which is today resistant cannot be used continuously, however, since the phage continuously undergoes genetic changes and culture strains that were not susceptible can become susceptible as new viruses develop against them. Successful starter culture selection becomes a matter of staying ahead of the change in the phage in the plant. Current methods of guarding against failure involve use of a rotation of blends of strains. The cheesemaker uses a number of different blends on different days. The blends are purchased from suppliers. The relationship between the strains in a blend has usually been determined by experimental methods and by trial and error so the phage produced (built up) against one strain or blend used one day will not attack the blend (or strains therein) used on the following day or the day thereafter. This is called rotation. The basis is trial and error and there is no assurance of success . . . and in practice the rotation system is only about 96% successful. In a large cheese plant 4% is a very significant loss.
A further factor to be considered is the effect of slight loss of starter culture efficiency due to phage attack on one or two of the strains sufficient to lower lactic acid production but still passable (as opposed to dumping the milk). This lowers the cheese quality and the change in acid (pH) could necessitate changes in the processing of the whey (a by-product) causing economic loss.
There are known tests for determining the phage susceptibility. The M17 agar plate method (the Australian Journal of Dairy Technology, June 1977, pp 63-64) is used in certain laboratories but is generally regarded as requiring too much skill and experience for the cheese factory. A simpler test involves the use of Bromcresol Purple (BCP) Milk in which the direct acidification of milk is wholly or partially inhibited by phage so that after incubating the resistant (usable) culture has turned yellow while the inhibited (not usable) culture is blue or green. Neither the BCP method nor the M17 agar plate method has been adopted for use in the factory.